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It’s easy to take bones for granted until you break one or are diagnosed with bone-thinning in old age.

In fact, you may not really consider your bones a resource that needs lifelong protection and nurturing.

But guess what? There are a few surprises lurking inside your bones, and youthful habits can even affect bones in old age. Read on to find out more about this living—yes, they’re alive!—tissue. (Photo Credit: Corbis)

They may feel like rocks, but bones are living things. In fact, a group of cells (called osteoblasts) are constantly churning out new bone, while a second set (called osteoclasts) destroys bone by gobbling it up like Pac-Man.

This constant tug of war between creation and destruction is called bone remodeling.

It’s the reason bones regenerate after a break, grow rapidly during youth, and, unfortunately, decline later in life when the balance tips toward destruction. (Photo Credit: Getty Images)

Playing sports, lifting weights, running, and almost any activity that moves muscle will trigger your bones to lay down more minerals and get stronger and more dense. Since bone density peaks around age 30 and then starts to slip away, the more you build in youth, the more you'll have to "spend" later. Think of it as your bone 401(k).

"When you have more money in the bank prior to bone loss, you won't reach a level of deficiency that sets you up for fracture," says Felicia Cosman, MD, clinical director of the National Osteoporosis Foundation. (Photo Credit: Corbis)

For your bones, that is. Young women who stop having a regular period get a steep hormone drop that mimics menopause, which leads to bone loss. Anorexia in particular can harm bones by halting menstruation.

"If women don’t menstruate or under-menstruate, it's like going through menopause in their 20s or 30s instead of their 50s," Dr. Cosman says. And the associated drop in estrogen causes the bone remodeling process to speed up and become imbalanced.

"The amount that dissolves ascends the amount that restores," she says. (Photo Credit: Getty Images)

The only problem is that sometimes toxic substances, like lead or mercury, can get lodged in the bone storage unit too. "Bone is a major storage pool for calcium and phosphate, and it stores substances like heavy metals in low levels for long periods of time," Dr. Cosman explains.

However, it is "very unlikely" that what our bones store—both good and bad—will at some point come out in large amounts and cause health problems, she says. (Photo Credit: Corbis)

This is a milder condition than osteoporosis, but alcoholism or habitual heavy drinking over time can cause calcium deficiency and the more severe bone thinning known as osteoporosis. Smoking, too, is a known risk factor for osteoporosis. (Photo Credit: Corbis)

How can a spare tire hurt your bones? Research suggests that premenopausal women who carry extra fat in their midsection are at increased risk for osteoporosis.

That’s because belly fat is different from the stuff that cushions your thighs or butt.

"[Abdominal fat] is really bad fat and very metabolically active," Dr. Cosman says. "It produces all kinds of hormones that can increase inflammation in the body, and the end result of inflammation is increased bone dissolution." (Photo Credit: Getty Images)

There are many bone-strengthening medications on the market. But they’re not for everyone.

Bisphosphonate drugs (Fosamax, Boniva, Actonel, Reclast) reduce fractures, but they can be tough on the stomach and have been linked in rare cases to fractures and serious jaw problems, Dr. Cosman says. (Dr. Cosman is a paid consultant for the makers of Fosamax and Reclast.)

She recommends getting reevaluated after three to five years to make sure you still need the medication. Changing drugs or dosages are options. Low-risk patients can even stop the regimen, Dr. Cosman says, but they have to be monitored very closely. (Photo Credit: Getty Images)

If the jawbone deteriorates or loses density, the result may be loose (or lost) teeth, receding gums, or ill-fitting dentures.

Dentists can pick up on osteoporosis by checking your regular dental X-rays and watching for related health problems, according to the National Institute of Arthritis and Musculoskeletal and Skin Diseases. (Photo Credit: Getty Images)

"Some highly fortified foods and drinks have as much—or more—calcium per serving as milk," she says.

Even fruits or vegetables without that much calcium can benefit bones (though research on this topic isn't conclusive). Fruits and veggies might help buffer acids in the body, and high acid content isn’t good for bones, Dr. Cosman says. (Photo Credit: Getty Images)

Although breast and other cancers top most women’s health-worries list, osteoporosis-related fractures should also be a major concern.

In fact, a woman’s risk for an osteoporosis-caused hip fracture is the same as her breast, ovarian, and uterine cancer risk combined.

A broken hip, which is most common in people older than 65, can be a potentially life-changing or even life-threatening event, and may require surgery and long recovery times. Some never fully recover from a broken hip and have to use a walker or wheelchair. (Photo Credit: Corbis)

While anyone can develop osteoporosis, your risk is much higher if you are white or Asian.

If you are black or Hispanic, your risk of developing the bone-thinning disorder is lower overall.

A survey found that between 13 percent and 18 percent of women older than 50 in the U.S. had osteoporosis in their hips, and that included about 20 percent of non-Hispanic white women, 5 percent of non-Hispanic black women, and 10 percent of Mexican-American women. Men have a lower risk of osteoporosis, but the risk is still higher in white and Asian men than in those with other ethnic backgrounds. (Photo Credit: Getty Images)